1. Technical Field
The present invention relates to optical orthogonal Frequency Division Multiple Access (OFDMA) networks and, more particularly, to a system and method for dynamically allocating sub-carriers between nodes.
2. Description of the Related Art
Metro core networks are frequently based on fiber optic rings, stemming from legacy Synchronous Optical Network equipment. These networks are often built on a Unidirectional Path Switched Ring structure, having two redundant optical channels that allow for extremely fast recovery in the case of disruption of service. A metro core network serves a relatively large area, with the rings often being hundreds of kilometers in circumference, and provides connection between the local access networks and the long-haul (or backbone) networks.
Prior art implementations of optical metro core networks have been built using time-based resource sharing, as seen in the use of network structures such as RPR, HORNET, and OBT. These resource-sharing schemes schedule transmission such that individual nodes transmit sequentially for a short period of time, using the full bandwidth of the fiber. However, this leads to inefficient use of the network's bandwidth, as it is not responsive to to individual nodes' Quality of Service (QoS) needs. For instance, if a particular node has little data in its queue, its time slot (and hence network bandwidth) will be underused.
There is a similar problem in the implementation of Passive Optical Networks (PONs) such as those used to provide access to homes and businesses. These networks use unpowered optical splitters to share a fiber optic link from a single Optical Line Terminal (located at the service provider) between a plurality of Optical Network Units (located at the end user). These systems typically use time-division to share the link between the users, which presents the same inefficiencies as when time-division is used in a metro core network.
It is therefore advantageous to implement a resource sharing scheme which allows all nodes to transmit simultaneously and which flexibly allocates bandwidth based on QoS needs. One implementation of a metro core network involves the use of an Orthogonal Frequency-Division Multiple Access (OFDMA) scheme. This technique uses a plurality of orthogonal (i.e., non-interfering) sub-carrier frequencies to serve a plurality of nodes. By splitting traffic between the sub-carriers, the bandwidth on the channel is increased without having to alter the infrastructure. In addition, different sub-carriers can be assigned to different nodes on the network, effectively splitting the available bandwidth and allowing all nodes to transmit simultaneously.
However, using a static allocation of sub-carriers leads to a problem similar to that presented in the time-division protocols. If a node is underusing its allocated sub-carriers, then that node's bandwidth is being wasted. In the wireless communications context, OFDMA has several proposed schemes for dynamically allocating sub-carriers between nodes in order to respond to QoS needs. However, these techniques are not effective in the optical domain due to its greater complexity, different fading channel, and low bandwidth flows.